Wireless communications systems of various kinds are well understood in the art. Some systems utilize frequency diversity to facilitate such communications. Multicarrier communication, such as orthogonal frequency division multiplexing (OFDM) is a particular way to support high data rate broadband systems that operate in a multipath environment. Generally, a multicarrier system can transmit on multiple carriers, and each of these carriers may also be referred to as a subcarrier. The OFDM approach divides a radio frequency channel into several narrower bandwidth subcarriers and transmits data simultaneously on each subcarrier.
Mobile reception over multipath channels is known to create variations in signal quality in both the time and frequency dimensions. For example, the root mean squared delay spread of the multipath channel strongly influences the coherence bandwidth of the channel, and the maximum Doppler spread (which is related to the mobile velocity) strongly influences the coherence time of the channel. Therefore, in a multicarrier communication system such as OFDM, the signal quality generally varies in two dimensions: time and frequency. Variation in frequency implies that different subcarriers may experience different signal qualities at the same time. Variation in time means that a particular subcarrier may experience different signal qualities at different times. The amount of change in signal quality between adjacent subcarriers is characterized by the subcarrier frequency spacing and the delay spread characteristics of the channel, while the amount of change in signal quality between adjacent OFDM symbol intervals is characterized by the OFDM symbol duration and the Doppler spread of the channel.
Multipath reception conditions are known to impact reception accuracy. Generally speaking, the more intense the multipath effect, the worse the reception conditions (though since this effect is frequency dependent, not all subcarriers of a multicarrier system are similarly affected). The multipath effect can result from a variety of causes. It is also known that with a moving context (i.e., when the transmitter, reception target, and/or target's environment is moving), the multipath effect can change very quickly. Hence, it becomes increasingly difficult with increasing speed of the target to have both the transmitting radio and the receiving target getting a reliable estimate of the instantaneous channel response. Also, at a given instant, the multipath effect can be very intense, resulting in a very complex channel frequency response.
To illustrate these points, consider the power versus time versus frequency graph presented in FIG. 1 (this figure depicts these parameters in a relative sense as between subcarriers and not as an absolute channel response; in terms of delay profile, it represents a “bad urban” channel with two clusters, each having a root mean squared delay spread of 1.2 μs). This data represents received power over time and over a number of subcarrier frequencies for a receiver moving at 27 kilometers per hour. Though signal perturbations in time do occur, for any given frequency these perturbations are relatively slow to occur. Referring now to FIG. 2, a similar graph is presented depicting data generated for a vehicle moving at 81 kilometers per hour. The perturbations in time are seen to occur more frequently. Similarly, and referring now to FIG. 3, the perturbations in time occur with even greater frequency for a vehicle moving at 135 kilometers per hour. In general, the rate of change of the channel power as a function of frequency is not significantly different with respect to the velocity because all of the examples have the same root mean square delay spread. If, however, the delay spread of the channel was increased, a corresponding increase in the rate of channel perturbations would be observed in the frequency dimension.
As noted, orthogonal frequency division multiplexing is a particularly apt choice for systems that must support communications in a harsh wireless communications environment. Consequently this approach finds use in, for example, cellular telephony systems. Generally, such a system is designed to accommodate mobile users in vehicles that are potentially moving at significant speed with respect to the transmission source. Some studies indicate, however, that in such a system, in fact a majority of the users at any given moment are not moving at a significant speed, especially in an urban environment. As a result, communications in support of a majority of the users will generally not experience a fast-varying channel in the time domain. This means that the overall system, optimized as it is to facilitate robust performance in a harsh environment for individual users, will actually exhibit an overall impairment of data system throughput when viewed with respect to all users because the system must accommodate some users in particularly difficult reception circumstances.
Adaptive modulation and coding schemes (where different modulation and/or coding schemes are selected to accommodate different situations) are also known and utilized to at least attempt to optimize data throughput as between a transmission source and a transmission target.
Such techniques have found use in the time dimension of wireless communications systems (for example, with code division multiple access systems or with time division multiple access systems). Unfortunately, use of such techniques in the time dimension lack the frequency domain aspects of orthogonal frequency division multiplexing. Consequently, such techniques may not be optimal for orthogonal frequency division multiplexing. (For example, using the same modulation on all subcarriers and adapting the modulation occasionally in the time dimension ignores the variability of the channel quality in the frequency dimension.)
Adaptive modulation and coding techniques have also been applied to the frequency dimension, for example, in Asymmetrical Digital Subscriber Line (ADSL) systems. However, the adaptive modulation and coding approach typically used in ADSL systems is only updated in time relatively infrequently because the channel between the transmission source and the transmission target remains almost constant over time (and also in part because of the considerable overhead that the technique imposes upon the system). Such a slowly adapting technique is not always applicable in a wireless cellular environment where the channel can change rapidly over time and frequency.
A need therefore exists for a way to improve upon overall system data throughput for a multicarrier communications systems such as a wireless orthogonal frequency division multiplexed communications system. Preferably any such improvement should not present a significant or undue loading requirement upon the system itself. Also, such a solution should not unduly delay the transmission of data in favor of overall improvement throughput. Further, any such improvement should be efficient in operation, relatively inexpensive, and effective.